The production of aluminum ingots and billets for fabrication into high quality aluminum products requires that the molten aluminum be relatively free of inclusions such as insoluble solid or immiscible liquid impurities. This is to ensure that the ingots or billets after downstream fabrication will meet the stringent requirements of high quality products such as rigid and flexible packaging materials, aerospace products (extrusions, sheet, plate, forgings), lithographic, automotive condenser tubing and bright trim. During the production of the molten aluminum insoluble impurities or inclusions are generated throughout the entire reduction, holding, alloying and casting processes.
In the primary electrolytic reduction process un-dissolved alumina; aluminum carbides; sodium aluminum fluoride from the electrolytic bath and γ-alumina skins are carried over as impurities and inclusions. In furnace holding and alloying stages magnesium aluminate spinet, magnesia, γ-alumina skins and furnace refractories are generated as inclusions or impurities. In the metal transfer during casting chloride salts, titanium boride clusters, eroded refractories and γ-alumina skins are generated as impurities or inclusions.
It is desirable to remove the inclusions in the last processing step prior to the molten aluminum being solidified into an ingot or billet through filtration. This has been done for many years in the industry through the use of a variety of technologies, including bed filtration and ceramic foam filtration as exemplified in Brondyke, K. J. and Hess, P. D., “Filtering and Fluxing for Aluminum Alloys”, Transactions of AIME, Volume 230, December 1964, pp. 1,553-1,556; U.S. Pat. No. 3,947,363, Issued Mar. 30, 1976, “Ceramic Foam”, Michael J. Pryor and Thomas J. Gray; and U.S. Pat. No. 4,343,704, Issued Aug. 10, 1982, “Ceramic Foam Fitter”, Jerry W. Brockmeyer.
Open-pore ceramic foam filters for use in aluminum cast houses were developed in the early 1970's with the first commercial application of the technology in 1974 to produce rolling ingots for fabrication into plate and sheet materials. Ceramic foam filters are monolithic, one-shot or disposable, filters that are used for a single cast. Filter pore size ranges from 4 to 28 pores per cm (10 to 70 pores per linear inch), which corresponds to pore with a diameter of about 0.036 to 0.26 cm. Ceramic foam filters are typically produced in square sizes ranging from 22.86 cm×22.86 cm×5.08 cm (9 in×9 in×2 in) to 66.04 cm×66.04 cm×5.08 cm (26 in×26 in×2 in) with a 17° edge bevel for seating into a refractory filter bowl as illustrated in FIG. 1. On the bevel edge a fiber paper gasket is fixed to provide lateral compression loading to hold the filter in place in the filter bowl and to prevent metal bypass around the edge of the filter. The fiber gasket material is typically about 0.317 cm to 0.476 cm (⅛ to 3/16″) thick and is typically comprised of silicate fibers. Vermiculite is oftentimes added to the gasket material, which expands during heating to increase the gasket pressure. Typical use time in the molten aluminum is 30 to 120 minutes.
During the 1980's there was rapid and widespread market acceptance of ceramic foam filtration technology for a broad range of high quality fabricated aluminum products including rigid packaging materials, lithographic sheet, aerospace products such as sheet, plate, forgings and extrusions; bright finish trim, condenser tubing, foil, architectural extrusions, foundry alloys and electrical conductor cable and wire. The subsequent rapid market acceptance and growth of the technology into aluminum cast houses of all types and levels of sophistication was due to the following reasons: ease of use and operator acceptance; operational flexibility; ability to drain after every cast; low variable operating cost; low capital installation cost; effective inclusion removal; and small foot requirements which equated to minimal floor space required for installation.
The earliest commercial ceramic foam filters were based on a chrome-alumina grain material, an aluminum orthophosphate binder and bentonite/kaolin additives to enhance slurry rheology. The chrome-alumina grain was relatively expensive and created a potential disposal problem due concerns regarding potential hexavalent chrome which is a known carcinogen. The chrome-alumina formulation was subsequently replaced by an “all alumina” formulation that incorporated alumino-silicate fiber and a mineral colloid, clay additive, while still utilizing the aluminum orthophosphate binder system of the chrome-alumina filters. This “all alumina” filter formulation has subsequently been widely used worldwide and has become the “industry standard” for ceramic foam filters used in aluminum cast houses for over 25 years. Despite the wide spread use of aluminum phosphate bonded alumina foam filter, there are several significant shortcomings to this filter formulation. The aluminum phosphate filters have poor thermal shock resistance, have a tendency to develop lateral compressive failures, lose strength during use due to attack of the aluminum phosphate bond; it has poor resistance to chemical attack and erosion of the filter structure. Further, phosphine gas can be generated from used filters, which complicates disposal.
The use of alumina grain in a ceramic foam filter would seem to be an obvious choice to anyone familiar with refractory materials used to contain molten aluminum and it's alloys. Alumina is relatively chemically inert in molten aluminum and it's common alloys, including those containing magnesium. It is also widely used as a grain material in refractories used in furnaces to both melt and hold molten aluminum alloys. Further, before the development of the one-shot disposable ceramic foam filter, tabular alumina bed filters were used to filter molten aluminum. Bed filters are large heated vessels containing un-bonded tabular alumina grains that are used for repeated casts over a period of several days or even weeks. The long exposure time of the molten metal to the un-bonded aggregate materials as in bed filters and refractories requires the use a chemically inert grain material such as alumina.
However, alumina has a relatively high coefficient of linear thermal expansion (8.0×10−6/° C.), and alumina monolithic shapes, such as ceramic foam filters, have poor thermal shock resistance due to the high thermal stresses generated by a combination of thermal gradients due to uneven heating and the high thermal expansion coefficient. During pre-heat and initial molten metal contact, the ceramic foam filter material may thermal-shock crack or spall and result in the release of filter material into the ingot or billet where it becomes an inclusion. In addition, when an alumina foam filter is restrained in a filter bowl during pre-heat and use, high lateral compressive stresses can be generated as a result of the high thermal expansion rate of the filter, leading to compression failure of the filter.
Aluminum orthophosphate (Al(H2PO4)3) is widely used as a refractory binder in the metals industry. It develops good green strength during drying at relatively low temperature, has low green shrinkage and develops good strength during subsequent firing. Aluminum orthophosphate is relatively inexpensive, widely available and requires relatively low firing temperature (1,100° C.) to obtain the final aluminum phosphate (AlPO4) bond. For these reasons, and because aluminum orthophosphate is relatively low cost, the material is widely used in the manufacture of ceramic foam filters for use in molten aluminum filtration. However, the resulting aluminum phosphate bond is subject to reaction with magnesium in many commercial aluminum alloys.
Magnesium is one of the most common alloy elements in commercial aluminum alloys. Magnesium in molten aluminum is highly reactive, has a relatively high vapor pressure and will easily penetrate into any refractory matrix where it will readily react with nearly all the common oxide materials. Aluminum phosphate is highly reactive to magnesium vapor in aluminum alloys and is not as stable as originally thought and disclosed in the Pryor and Brockmeyer patents. Instead, the material is subject to reduction by magnesium:AlPO4(s)+4Mg(g)AlP(s)+4MgO(s)
Because, the aluminum phosphate is a contiguous part of the filter matrix, the degradation of the aluminum phosphate bond leads to reduced strength or “softening” of the filter during use. The corrosive attack of the bond-phase is of an inter-granular nature, compromising the filter structure and potentially subjecting the filter to premature failure in use. The reaction occurs at even relatively low temperatures, just above the aluminum liquidus temperature, and increases rapidly with time, magnesium content and temperature. Metallurgical analysis of used filters using both optical and scanning electron microscopy confirms the degradation of the aluminum phosphate bond. FIG. 3 shows the intergranular attack of an aluminum phosphate bonded alumina filter. The above reaction results in the molten aluminum wetting into the filter structure and increased corrosion of the filter structure. Corrosion of the filter structure results in the release of alumina grain and aluminum phosphide particles into the molten aluminum where it becomes inclusion material in the alloy melt. In addition the aluminum phosphate bond does not protect the alumino-silicate fiber in the filter matrix, which is also chemically attacked.
The aluminum phosphide that remains inside of the filter after corrosive attack becomes a potential hazard in subsequent handling and disposal of the used filter. When used filter material comes into contact with atmospheric water vapor or in direct contact with water, phosphine gas will form according to the reaction:2AlP+3H2O2PH3+Al2O3.
Phosphine gas is a highly flammable and toxic gas. As a result used filters may require special handling.
The aluminum phosphate bond also contributes to poor thermal shock resistance. After firing the aluminum phosphate has a berlinite crystal structure, which goes through a structural phase transformation with a 2 to 3% volume increase in the 80 to 180° C. temperature range. This volume change results in abrupt expansion lowering the materials thermal shock resistance and increases the compressive lateral stress in the filter body. FIG. 2 shows thermal expansion of an aluminum phosphate bonded alumina filter and the low temperature phase transformation.
The following are the ideal material requirements for a ceramic foam filter material:                1. High Thermal Shock Resistance—the material must not crack or spall during pre-heat or molten metal contact. The material should have low thermal expansion to minimize lateral compressive stresses while seated in the filter bowl.        2. Corrosion Resistance: The filter material should not react significantly in the intended application range (time, temperature, alloy content) and must remain non-wetted by molten aluminum and it's common alloys.        3. Adequate bending and compressive strength        4. Economical to produce.        5. Filter material after use must be safe to handle and dispose.        6. Low Density or light weight for the casting pit operator to easily handle        
There has not yet been a filter material that provides all of these features. Provided herein is such a filter.